A glycoprotein isolated from rabbit hepatocytes (HBP) which has the ability to bind specifically desialylated glycoproteins is also capable of inducing mitogenesis in human desialylated lymphocytes. The immunogenic properties of HBP, the first lectin of mammalian origin to be recognized, is being studied further. It has been shown that isolated HBP has the ability to increase the cytotoxicity of human lymphocytes against Chang target cells to the same degree as the plant lectin phytohemagglutinin. HBP did not increase the cytotoxicity of intact lymphocytes. In addition, it has been shown that desialylated lymphocytes induced significantly more cytotoxicity in isolated rabbit hepatocytes, which bear HBP on their surface, than did intact lymphocytes, and that this phenomenon could be blocked by asialo-orosomucoid (a desialylated glycoprotein) but not by intact orosomucoid. These findings suggest that the cytotoxicity induced by membrane-bound HBP is similar in mechanism to that induced by the isolated protein and that the induced cytotoxicity probably depends on the interaction between HBP and exposed galactose residues in desialylated lymphocytes.